Existing blown film processes involve blowing a film tube or bubble into a cylinder and maintaining the cylindrical form until the tube is converted to a web. The tube is blown and drawn out from a die and rotated at a substantially constant speed. Prior to conversion, the tube is collapsed into a two ply sheet to permit the blown film to be further treated and cut into a web. Recent representative examples of collapsed tube blown film processing are found in U.S. Pat. No. 4,189,288 to Halter and U.S. Pat. No. 4,699,580 to Co. Collapsing boards, rollers, or other tube restricting devices are used to seal and drive the film tube.
However, these closed nip processes have a propensity for wrinkling the film during collapsing. The folds formed during collapse of the tube are permanently creased and the portions around the folds are usually cut off and discarded. Also, the laminating force provided by the nip often causes the two film layers to stick together, thereby eliminating the possibility of coating or coextruding any material that this laminating would damage. Moreover, as the tube is closed, the nip blocks access to the inside of the tube, severely restricting processes such as internal tube cooling or coating. This causes blown film dies to be very complicated and expensive when the interior of the tube is to be treated; air channels must be machined through the extrusion die. Even with these air channels, air passing through the channels is warmed by the die as the die is cooled by the air, both of which are disadvantageous to the web forming process.
In U.S. Pat. No. 3,342,657 to Dyer, an apparatus for forming a thermoplastic film is disclosed which forms the film in a tube and cuts the tube into webs without closing the tube. However, although this system uses an annular rotating extrusion die, the tube is not blown. Rather, the tubular film is pulled over mandrels within the tube which serve as cooling and heating devices. Additionally, the tube is stretched and oriented between the mandrels. These mandrels block access to the interior of the tube during tube formation and no devices control the external diameter of the tube. Moreover, rollers must be placed inside the tube to transport the tube downline.
Japanese Kokai Publication No. JP63-151429 to Goto is directed to a method of producing flat films using an open tube process. However, the apparatus appears to be inoperative as shown. Additionally, there is no control over the size of the tube and no way to prevent the tube from contacting the internal stage barriers and stopping operation. Changing the internal air pressure is the only method disclosed of varying the diameter of the tube. However, this simplistic approach is not sufficiently precise to produce commercially acceptable, uniformly thick webs and air leakage further complicates operation.
Additionally, the slitting devices and tube stabilizing apparatus components of known web forming systems suffer from numerous drawbacks. Known slitting devices for cutting a flat web from a tube slit in the machine or longitudinal direction or with a slight, oscillating bias, and offer very little spreading of the web caliper variation. This can lead to hard bands in wound rolls, which result from nonuniform web caliper.
Numerous types of apparatus for stabilizing a blown film tube are known. Sizing cages include a cylindrically-shaped shell of small diameter rollers must be positioned above the tube frost line to prevent the tube from sticking to the rollers and creating surface defects. However, this location virtually eliminates any size limiting features of the cages as the tube is already solidified. Internal mandrels over which the tube is physically stretched to a final diameter require an internal heat removal mechanism which makes startup and operation difficult and increasing costs. Contact between the tube and the mandrel also causes surface defects.
Rings of air chambers are described in U.S. Pat. No. 3,976,732 to Herrington and U.S. Pat. No. 4,728,277 to Planeta. In Herrington, a plurality of air rings are positioned around the blown tube and have differing diameters to mechanically define the diameter of the blown tube. The rings form a conical, rather than cylindrical shape which do not provide as stable a film diameter. Planeta discloses a film handling device in which a plurality of stabilizing devices which create axially aligned air rings control the shape of a blown tube. Each device uses two oppositely moving air streams parallel to the tube wall to create a low pressure zone to hold the film in position. This does not produce a sufficiently stable film diameter. Additionally, these complex air-based systems rely on the air to impinge on and flow around the blown tube rather than providing a continuous cushion for the tube.
U.S. Pat. No. 4,655,988 to Shinmoto et al. discloses a vacuum system for regulating the diameter of an extruded, blown tube. In this system, a plurality of air-introducing arms are twisted like a vortex to form a structure in which the internal diameter is physically adjustable like a diaphragm. While the air provides a buffer between the arms and the film, it is the changing internal diameter of the physical structure which regulates the size of the tube.
Size feedback systems measure the diameter of the blown film tube above the frostline and vary the amount of air in the tube interior to control the diameter. These devices monitor the tube diameter with sonar or optical sensors. However, as the tube is formed to its final diameter, this method involves compensating for an error in size which has already occurred. This results in a tube having varying diameter depending on each response of the air control system. Additionally, in all known systems, this is accomplished as part of a sealed tube operation in which the tube is sealed typically at a two roller nip. In these systems, the tube diameter expands or contracts as a result of the air volume and pressure change and measurable diameter changes are required to attain a correcting action. This is not fast or accurate enough to permit the use of these systems with diameter control with open tube systems.
Moreover, these size feedback systems for controlling diameter are inadequate when an internal, imperfect seal or plug is used instead of the two roller nip. The tube diameter responds to very slight changes in internal pressure and volume. None of these pressure control systems can respond quickly and accurately enough to prevent diameter changes due to seal leaks. These systems are therefore unacceptable for use with the imperfect seals of open tube processes. Furthermore, in these methods, process disturbances, such as changes in polymer properties or temperature can result in a larger diameter tube and a slightly lower frost line height.